Essentially, this is a love story mixed up with adventure and some fights. It's a little bit different from what I've already read from Kaori Ozaki. Meteor Methuselah looks like an attempt on shounen territory but without abandon the theme of love that generally accompanies the author throughout her other works.
The protagonist, Rain, deals with the fact he's unable to die. He's 620 years old. The behavior from him is not we would expect, though. Rain is very relaxed about it. He chooses to embrace his situation even though he has experienced the loss of people he loved in the past. When Rain encounters Machika - in the beginning as an enemy - they start their path towards a love story.
Machika is the active type of character which balances well with Rain laziness. She's strong, decided. The lovable aspect of her is that we can see her development, much more than the other characters. She's 14, so a teenager. It's possible to see her childish thoughts towards Rain. Kaori Ozaki didn't treat her as an adult as much shounen does when we have a character that should be 14 or 15. Still, they have these fully developed bodies and mind turning them into an ageless character, consequently stealing a little bit of their identity.
Machika is a teenager, and we can tell that either in her behavior as in her body. She's so small compared to Rain that it's weird to imagine them as a couple. It looks wrong. I think the author nailed in that regard. Different from other works of her, the main couple is not evenly matched in their age. She's 14 years old with an adult "boyfriend." (In the end, I rationalized that in the past, in many cultures, this was the age when women were considered adults, so in the context of the manga should be acceptable. However, in the modern world, this is too much, so, for me, it was impossible not to find that weird to the end). Things got complicated as she starts to mature – only in her mindset aspect since the whole story happens in more or less one year. As the story goes on, she turns more into a partner for Rain, rather than some child he's taking care of. On the other side, we see Rain been more human, less the immortal and wise being, beyond our comprehension. So, it's a great dynamic between them, where the more Machika grows, the more of Rain's weakness we see.
The entrance of the third main character turns things more interesting. Yuca starts as an enemy, the antagonist of the story. The natural threat that needs to be defeated or solved by the main characters, but as some events happen, his role in the story changes significantly, and so he ends up being the third part in the Rain-Machika dynamic. Sorry for being vague about him, but since Yuca appears only mid-way through the series, it's difficult to talk about him without giving major spoilers. Only knows that Yuca's presence helps Meteor Methuselah goes past the average shounen story. We don't have a clear antagonist, some force to be destroyed for the sake of humanity. It's a story where characters' interaction is way more important than the big events that happen, which is a characteristic of the genre.